Antibodies against at least one T-cell lymphotropic virus were detected in 31/163 (19%) chimpanzees Pan troglodytes sp. Serological investigations indicated that virus transmission was occurring in these animals at the rate of one or more seroconversions per annum. In order to determine the possible origin of the virus or viruses DNA was extracted from peripheral blood lymphocytes of the chimpanzees for testing. Provial DNA was detected in 20 of 22 DNA samples using four polymerase chain reaction assays targeting three different regions of the simian T-cell lymphotropic virus type 1 (STLV-1) genome. DNA from antibody negative animals (n=5) proved negative for amplification. Phylogenetic analyses demonstrated that three variants of STLV-1 were present in the colony. These cladistic variants have been associated with West African subspecies of the African green monkey Cercopithecus aethiops sabaeus; West African and Central African chimpanzees respectively. No human T-cell lymphotropic virus proviral DNA was detected.